At the part when you have to block the double doors with your axe after escaping the Western L Block, if you do it before the doors actually close, the doors will open back up and the NPCs and doors will be stuck. I don't know if this was already reported, but I just looked through these pages on this topic and didn't see the same issue.
I thought there was a way to force a side quest to start through the console, but I couldn't find where I thought I saw that. The thing is, Bryan's first side quest won't start anymore after I have reloaded a save. I reloaded because I did initially have his quest in hand, but I didn't finish it quick enough and when I got back, it triggered the
sequence.

He doesn't even get to France. Jake died/zombified in the Arctic, and the player is posthumously informed that France has been zombified.
So Jake died in the end? This changes the entire plot of Underhell.

I'm pretty sure he just said that.
But anyway, it would be nice if stealth was more easier. I personally like to sneak if there's an option, but it was always just so hard. I even cheated to use the "save" console command, and a total count of the first stealth level's restarts due to alerts was something like 15-20.

It's not like there are houses that are built for just one or two people nowadays.
The playground had to have been there long before Jake married. According to Jake's calender, Jake's wife moved in and didn't even survive a week.
Like said above, Mrs. Hawkfield didn't survive the first week of living there, so I don't think they'd even have enough time to check if she was ever pregnant.
That room is obviously WIP if it only has a lamp and a desk.
There are not enough facts to prove that there is a third member. All of these "facts" are just hunches.
Jake simply and plainly says that the door is locked. Not because he doesn't want to go in there.
Nice theory dough.